As the day progresses so does the wallpaper; at noon your wallpaper is a bright vista but come 9PM it looks dusky; midnight it is lit only by stars.

You get the idea.

The pack could, undoubtedly, do with a few more images so that the progression is more pronounced than currently (it comes with 8 images that change roughly every few hours due to there being less images used during the night and more during the day.)